Natural gas is a valuable, environmentally-friendly energy source. With gradually decreasing quantities of clean easily-refined crude oil, natural gas has become accepted as an alternative energy source. Natural gas may be recovered from natural gas reservoirs or as associated gas from a crude oil reservoir. Indeed, natural gas for use in the present process may be recovered from any process which generates light hydrocarbon gases.
In many offshore areas where hydrocarbon resources may be found, there are generally no natural gas pipelines available. As a result, the developer of hydrocarbon resources must either build expensive facilities to re-inject the gas back into the ground, build new pipelines to take the gas to distant markets, or construct expensive liquefied natural gas (LNG), gas to liquids (GTL) or similar facilities to liquefy or re-form the natural gas for transport to distant markets. Flaring of the produced natural gas does not take advantage of the gas as an energy source and is no longer a suitable disposal method for obvious environmental reasons. There is a need for a relatively simple and inexpensive process to produce, store and transport natural gas from offshore fields.
The discovery of clathrates, also known as hydrates, is credited to Humphrery Davey and Michael Faraday, in the early 1800's. Hereinafter, we will use the common word ‘hydrate’ to mean clathrates, gas hydrates and inclusion compounds. Faraday published a paper on chlorine hydrates in 1823. For almost a century, hydrates remained essentially an intellectual curiosity. Villard, De Forcrand and others in France conducted extensive work in determining what components form hydrates and under what conditions of pressure and temperature. In the 1930's, Hammerschmidt realized that the ice-like blockages that formed at temperatures above 0° C. (32° F.) in the increasingly high pressure natural gas pipelines was due to the formation of hydrates. From that point, scientific attention was focused on the prevention and decomposition of natural gas hydrates. Much of the work was done at the University of Michigan under Professor Katz. In Europe, von Stackelberg was at the same time examining hydrate structures using x-ray diffraction. In 1959 in the Netherlands, Van der Walls and Platteeuw were the first to publish a rigorous thermodynamic model for calculating the conditions at which hydrates form. Research on natural gas hydrates has increased in the last few decades both to understand the geophysical phenomenon of naturally occurring methane hydrates in arctic areas and ocean bottoms as well as the general production, storage, transportation and decomposition of natural gas hydrates. Some investigations have also been made into production and decomposition of hydrates as a means to desalinate seawater.
Hydrates are metastable non-stoichiometric crystalline, ice-like solids composed largely of hydrogen-bonded lattices (3-dimensional cages) of hydrogen oxide (water) molecules that contain within their cages other small molecules (hydrate formers). The small molecules enter the lattice and stabilize it. The water molecules are referred to as the “host” molecules and the other molecules are “guest” molecules or ‘hydrate formers’. An interesting aspect of the hydrates is that there is typically no bonding between the guest and host molecules. The guest molecules can freely rotate inside the host cages.
Gas hydrates usually form one of three basic crystal structures known as Structure-I, Structure-II and Structure-H. These structures are able to host guest molecules with molecular diameters ranging between 2.2 and 7.1 angstroms. More specifically, guest molecules can be methane, ethane, propane, isobutane, carbon dioxide, hydrogen sulfide, nitrogen, chlorine, 2-methylbutane, methylcyclopentane, methylcyclohexane, cyclooctane and the like, and mixtures thereof. Normal butane is a special case. Although pure normal butane will not by itself form a hydrate, it can form hydrates in mixtures with other guest molecules.
Hydrates form when a sufficient amount of water and hydrate former are present under the right combination of temperature and pressure, which can include temperatures above the freezing point of water 0° C. (32° F.). One cubic meter of methane hydrate can contain, for example, 171.5 standard cubic meters of methane at near-atmospheric pressure. Hydrates are stable at high pressures (usually but not always greater than atmospheric pressure) and are poor conductors of heat.
Below, Table 1 illustrates experimental data for natural gas component quadruple points (Q1, Q2) used in a hydrate phase diagram. From such a phase diagram, the right combination of temperature and pressure for hydrate formation can be determined. Note that these quadruple points may vary depending on gas concentration/combination, water purity, etc.
TABLE 1ComponentT(K), P(MPa) at Q1T(K), P(MPa) at Q2Methane272.9, 2.563No Q2Ethane273.1, 0.530287.8, 3.39 Propane273.1, 0.172278.8, 0.556Iso-Butane273.1, 0.113275.0, 0.167Carbon Dioxide273.1, 1.256283.0, 4.499Nitrogen 271.9, 14.338No Q2Hydrogen Sulfide272.8, 0.093302.7, 2.239
Hydrate technology is being developed for production, storage and transportation of natural gas, particularly for remote fields with associated or non-associated natural gas. Hydrate technology may be competitive with liquefied natural gas and other natural gas technologies as a means to commercialize natural gas resources.
Several barriers to commercially viable hydrate production exist, including: the need for large amounts of fresh water; the slow formation rate of hydrates unless significant amounts of turbulence or agitation are present; the high pressures required; and, the high latent heat of formation which requires significant amounts of heat to be removed during the process. The formation of hydrates in a quiescent system is extremely slow at hydrate forming temperatures and pressures. Attempts to improve hydrate production include “rocking” the apparatus, or by mechanical stirring of the contents. As a consequence, many of these processes are necessarily of a batch nature. Another partial deficiency of hydrate production is that free water (not bound by the hydrates) remains between the hydrate particles in the interstitial spaces. Even an apparently solid hydrate mass can contain large amounts of free water. It is possible for more free water to be present in hydrates than bound water. This leads to storage and transportation inefficiencies.